1. Technical Field
The present invention relates to a distortion compensation amplifier which compensates for nonlinear distortion of an amplifier with a pre-distortion scheme, and more particularly, to a distortion compensation amplifier which accelerates the convergence of distortion compensation and improves the performance of distortion compensation.
2. Description of the Related Art
For example, a base station apparatus provided for a mobile communication system needs to transmit a radio signal to the position of a physically remote mobile station apparatus, the signal needs to be amplified significantly by an amplifier. However, the amplifier is an analog device and its input/output characteristics represent a nonlinear function. Especially, after the amplification limit called a saturation point, the output power is substantially constant even when the power input to the amplifier is increased. The nonlinear output produces nonlinear distortion.
In a signal for transmission before amplification, a signal component outside a desired signal band is reduced to a low level by a band limiting filter. In a signal after passage through the amplifier, however, the nonlinear distortion is produced to leak a signal component outside the desired signal band (to an adjacent channel). For example, since the base station apparatus has the high transmission power as described above, the magnitude of the power leaked to the adjacent channel is strictly specified, and how to reduce the power leaked to the adjacent channel presents a serious problem. A pre-distortion scheme is one of distortion compensation schemes and has been replacing a feed-forward scheme and becoming predominant since great importance has been put on the amplification efficiency in recent years.
To reduce the distortion to a low level with the pre-distortion scheme, a pre-distorter previously supplies a signal with the distortion of inverse characteristics of AM/AM characteristics and AM/PM characteristics of the amplifier to momentary power, and the signal supplied with the inverse characteristics is input to the amplifier, so that the output from the amplifier has linear characteristics with nonlinear regions canceled out. Thus, the distortion can be reduced to a low level while high efficiency is maintained.
FIG. 3 shows a configuration example of an amplifier with distortion compensation (distortion compensation amplifier) using the pre-distortion scheme of distortion power detection type.
An example of operation performed by the distortion compensation amplifier of the present example will be shown.
A power detection section 101 detects the power (or the amplitude) of an IQ digital signal input thereto, and an address calculation section 102 calculates a reference address for a distortion compensation table 103 formed of a memory or the like. The distortion compensation table 103 has a table stored therein for performing the distortion compensation with the pre-distortion scheme and outputs a distortion compensation value associated with the value calculated by the address calculation section 102 to a pre-distorter section 104.
Inverse characteristics of nonlinear AM/AM characteristics and AM/PM characteristics of an amplifier to be compensated for and correction values for compensating for distortion due to a memory effect are stored in the distortion compensation table 103. In the present example, the distortion compensation table 103 is stored after deployment to an amplitude table and a phase table within a control section 115 and then rectangular coordinate conversion to an I, Q table.
The pre-distorter section 104 performs complex multiplication of the reference result from the distortion compensation table 103 and the input signal. The resulting signal previously supplied with the distortion in the pre-distortion scheme is converted into an analog signal through a D/A (Digital to Analog) converter 105 and is converted into an RF (Radio Frequency) signal by a frequency conversion section 106. Then, the signal is amplified by an amplification section 107 into an output signal including no distortion which is output from an antenna 108.
Part of the output signal is taken out by a directional coupler 109, frequency-converted as a feedback signal into a signal of IF (Intermediate Frequency) band by a frequency conversion section 111, converted into a digital signal by an A/D (Analog to Digital) converter 112, and converted from time-base data to frequency-base data by an FFT (Fast Fourier Transform) section 113. Distortion power is calculated by a distortion band power calculation section 114 from the data after the FFT processing. The control section 115 updates the distortion compensation table 103 so as to reduce the feedback distortion power from the distortion band power calculation section 114, and performs adaptive control in order to follow temperature changes or the like.
FIG. 4 shows a configuration example of an amplifier with distortion compensation (distortion compensation amplifier) using the pre-distortion scheme of input/output signal comparison type.
An example of operation performed by the distortion compensation amplifier of the present example will be shown.
A power detection section 201 detects the power (or the amplitude) of an IQ digital signal input thereto, and an address calculation section 202 calculates a reference address for a distortion compensation table 203 formed of a memory or the like. The distortion compensation table 203 has a table stored therein for performing the distortion compensation with the pre-distortion scheme and outputs a distortion compensation value associated with the value calculated by the address calculation section 202 to a pre-distorter section 204.
Inverse characteristics of nonlinear AM/AM characteristics and AM/PM characteristics of an amplifier to be compensated for and correction values for compensating for distortion due to a memory effect are stored in the distortion compensation table 203. In the present example, the distortion compensation table 103 is stored after deployment to an amplitude table and a phase table within a control section 217 and then rectangular coordinate conversion to an I, Q table.
The pre-distorter section 204 performs complex multiplication of the reference result from the distortion compensation table 203 and the input signal. The resulting signal previously supplied with the distortion in the pre-distortion scheme is converted into an analog signal through a D/A converter 205 and is converted into an RF signal by a frequency conversion section 206. Then, the signal is amplified by an amplification section 207 into an output signal including no distortion which is output from an antenna 208.
Part of the output signal is taken out by a directional coupler 209, frequency-converted as a feedback signal into a signal of IF band by a frequency conversion section 211, converted into a digital signal by an A/D converter 212, and provided as an IQ signal by a quadrature detection section 213. For the I and Q feedback signal, a feedback level correction section 214 adjusts the level between the input signal and the feedback signal, a feedback delay correction section 215 adjusts the delay between the input signal and the feedback signal, a feedback phase difference correction section 216 adjusts the phase difference between the input signal and the feedback signal, and then the signal is input to the control section 217. The control section 217 detects an error component from the input signal and the feedback signal, updates the distortion compensation table 203 so as to reduce the error component, and performs adaptive control in order to follow temperature changes or the like.
See prior art reference: JP-A-2004-343496.